Mode I fracture toughness is a signifcant parameter for investigating the failure behavior of rocks. This parameter has several applications in many different felds, such as mining and tunneling.As the determination of fracture toughness by conducting tests is very time consuming and expensive, using empirical relationships is recommended in this regard. In this study, uniaxial compression, the Brazilian and three-point bending tests were conducted on the specimens of limestone, sandstone, tuffte, lithic tuffs, andesite and travertine. This study aims to determine an empirical relation that enables us to estimate mode I fracture toughness of rock using the brittleness index. The study indicates that of the three brittleness indices (B1, B2 and B3), mode I fracture toughness only has a relationship with brittleness index B1 which has the coeffcient of determination R2= 0.995. This relationship, which is an exponential one, reveals that as the brittleness index increases, the magnitude of mode I fracture toughness increases non-linearly. The proposed relationship equivalent was compared with that of Kahraman and Altindag, and the root-mean-square error was calculated in two equations. Given this calculation, it can be said that the proposed equation, with higher precision than the one proposed by Kahraman and Altindag, can be used to estimate mode I fracture toughness of rocks.